This disclosure is related to the field of well logging instruments having sensors that make measurements usable to generate an equivalent of a visual image of a wall of a wellbore through which the instrument is moved. More specifically, the disclosure relates to methods and systems for processing such measurements to automatically identify certain types of geologic features from the measurements. This section is intended to introduce the reader to various aspects of the technical field of the disclosure that may be related to the subject matter described and/or claimed below. This section is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this context, and are not to be construed as admissions of prior art.
Well logging instruments are used in wellbores drilled through subsurface formations to make, for example, measurements of selected physical parameters of the formations to infer properties of the formations surrounding the wellbore and the fluids in void spaces in the formations. Well logging instruments known in the art include electromagnetic tools, nuclear tools, acoustic tools, and nuclear magnetic resonance (NMR) tools, though various other types of tools for evaluating formation properties are also known.
Well logging instruments may be deployed in and moved along the interior of a wellbore on an armored electrical cable (“wireline”) after the wellbore has been drilled. Present versions of such “wireline” well logging instruments are still used extensively. However, as the demand for information during the drilling of a wellbore continues to increase, measurement-while-drilling (MWD) tools and logging while drilling (LWD) instruments have been developed to fulfill such demand. MWD tools are generally defined as those making measurements of drilling parameters such as axial force (weight) on a bit used to drill the wellbore, torque applied to a drill string, wellbore temperature, wellbore fluid pressure, and well trajectory direction and inclination. LWD instruments are generally defined as those which make formation parameter measurements such as electrical resistivity, fractional volume of pore space in the formations (“porosity”), acoustic velocity, density, neutron hydrogen index and/or capture cross-section and NMR relaxation time distributions, among other measurements. MWD and LWD instruments often have sensors similar in nature to those found in wireline instruments (e.g., transmitting and receiving antennas, sensors, etc.), but MWD and LWD tools are designed and constructed to operate in the harsh environment of wellbore drilling.
Well logging measurements may be processed to form images. Such processing may include plotting values of one or more well logging measurements in the form of gray scale or color scale with respect to both axial position in the wellbore (measured depth) and circumferential orientation within the wellbore. Logging-while-drilling (LWD) images acquired in highly inclined or horizontal wellbores may be characterized by various features that are sensitive to formation geologic structure near the wellbore. In well log data processing known in the art, image features commonly referred to as “sinusoids”, “bulls-eyes”, or “reverse bulls-eyes” may extracted from the images manually. However, manual feature extraction is time consuming and prone to user bias. This is of particular concern in highly inclined and/or horizontal wells, where small errors in determining formation layering angle with respect to horizontal (“structural dip”) may translate into large errors in calculated formation reservoir volumetrics. See, for example, Q. R. Passey et al., Overview of High-Angle and Horizontal Well Formation Evaluation: Issues, Learnings, and Future Directions, SPWLA 46th Annual Logging Symposium, Jun. 26-29, 2005. Furthermore, “bulls-eye” features have been observed extending for hundreds of feet in measured depth (axial length along the wellbore). It is therefore important to account for changes in both wellbore trajectory inclination and geodetic or geomagnetic direction (“azimuth”), and formation dip/azimuth, in the structural interpretation of such formations.